This invention relates to an arm for coupling a driving mechanism to a driven mechanism for the purpose of transmitting linear motion between the two mechanisms in both a forward and reverse direction. More particularly, it relates to a coupling arm that flexes, without any backlash in the flexing mechanism, to reduce the forces on the moving components caused by misalignment.
There are many types of apparatus where a driving mechanism moves with a linear motion in both a forward and a reverse direction to move a driven mechanism to a desired position. One such example is an XY positioning table, wherein the table can be moved in the plus and minus X direction and the plus and minus Y direction to position an object on the table. Such XY tables are widely used, e.g., in automated milling equipment, photoplotters, electron beam systems used to expose integrated circuit patterns, equipment for automatically inserting electronic components in printed circuit boards, etc.
An XY positioning table typically consists of two movable platforms. Two parallel rails, or slides, are mounted on a base and one of the platforms is mounted on the rails by some suitable friction reducing mechanism, such as ball bearings, roller bearings, etc., near each end of the plaftform. A second set of parallel slides is mounted on the first platform and the second platform is mounted on these slides, again using some suitable friction reducing mechanism. The two sets of slides typically are mounted at right angles to each other such that one platform can move in a defined X direction and the other in a defined Y direction.
Each platform is mechanically connected to a drive motor by some type of coupling arm. The drive motor and associated driving mechanism can cause the platform to which they are connected to move in either the plus or minus direction. Usually, an XY table is electronically controlled and a position sensing detector (or detectors) provides a position signal to a servo controller which automatically moves the two platforms in their respective directions such that the object mounted on the top platform is moved to the desired XY position.
Unfortunately, there is always some misalignment in a mechanical system of the type described above. This misalignment is typically the result of tolerance build-up in the fabricating and assembly process. For example, the slides may not be exactly straight, the slides may not be exactly parallel, the driving mechanism may not drive exactly in the same line in which the platform is moving, etc. Such misalignment may not necessarily be linear along the travel of the platform. For example, the slides may have slight bows in them such that at some points there is more friction between the slides and the bearings than at other points. The result of this friction is wear on the slides and the bearings. If enough wear occurs, the positioning accuracy of the XY table can be significantly affected.
In the prior art, this misalignment is handled in one of two ways. If the misalignment is negligible, then no corrective action is taken. This would be the case when the position error caused by the eventual wear is less than the rated positional error of the XY table.
However, in a "precise" XY table, corrective action is always taken. The XY table in an electron beam system used to expose circuit patterns in the fabrication of integrated circuits is an example of a precise XY table. The servo controller, using a laser interferometer as the position measuring device, can position the XY table to an accuracy of 0.20 microns (0.8 microinch). Circuit patterns, with dimensions and spacings on the order of a micron, are exposed on masks. A plurality of different masks are required to fabricate an integrated circuit and the patterns on each mask must be in precise alignment with each other.
A precise XY table is built using the tightest fabrication and assembly tolerances that are practical. As explained previously, there will still be misalignment within the mechanical assembly but it will be reduced to a practical minimum. If the driving mechanism is rigidly coupled to its platform, additional forces, and therefore friction and wear, are generated as the platform is forced through the points of misalignment. The result of this wear is that a given point on the positioned XY table, will move with respect to its location before the wear.
To assist in understanding how the location of a point on an XY table can move due to wear while the table is being positioned with an accuracy of 0.02 microns, consider the following example, which is given only for illustrative purposes: The lower platform moves in the X direction and the X driving mechanism is one degree out of alignment with the platform. After a sufficient amount of time, the slides of the X platform have worn until the platform is in alignment with the driven mechanism, i.e., rotated one degree from the original position. The upper platform, which moves in the Y direction, has no misalignment, and therefore no wear, is thus also rotated one degree. The servo controller, using the laser interferometer to measure position, will position the table to the desired XY coordinate with an accuracy of 0.02 micron. However, the electron beam is deflected in a coordinate system that is independent of the coordinate system of the XY table. Thus, the pattern exposed by the electron beam, about the XY point at which the table is positioned will be rotated one degree from what it was before the wear occurred.
While the above example is highly improbable, it illustrates the problem of the wear caused by the misalignment. Since, in reality, the misalignment is not linear, the wear will occur at localized points along the X and Y rails. The resulting errors in position can, of course, be in any direction, X, Y, or Z, or combinations thereof, at the points of wear.
Since misalignment is inherent in an XY table, the correction mechanism used in the prior art is to allow the coupling mechanism which couples the driving motor to the platform to "float". A floating coupling arm can transmit linear motion but can also flex, or bend, in at least one axis, to reduce the forces applied by the driving mechanism at the points of misalignment.
Typical flexing mechanisms used in the prior art are the hinge, the U-joint, and the ball and socket. Regardless of the type of mechanism used, they comprise two parts mechanically coupled together in such a way as to be able to move with respect to each other in one or more axis while transmitting a linear motion.
Unfortunately, since it is physically impossible to make the parts of the flexing mechanism that mate with each other exactly the same size, such floating mechanisms, or flexures, always have an inherent backlash. That is, when the direction of motion is reversed, one of the two parts will move a finite amount before the mating means causes the other part to move. Disadvantageously, this backlash causes wear in the flexing mechanism. As the wear in the mechanism increases, the servo system response time can increase, depending upon the type of servo system used, as it moves the mechanism back and forth through the backlash while attempting to move the platform to the desired position. The backlash can also cause an error in position of the object being moved, again dependent upon the type of servo system used, by reducing the positional accuracy of the servo system. When the wear in the flexing mechanism becomes too great, it must be replaced.
From the above discussion it is apparent that a need exists in the art of precise XY positioning tables for a coupling arm between the driving mechanism and the driven mechanism that has no backlash and that can flex to reduce the forces caused by the misalignment inherent in the two mechanisms.